The food industry in particular, requires a non-invasive, non-destructive temperature measuring technique for process monitoring, product safety and quality control. Typical requirements are to check that a food product has been cooked, or at least partly cooked, to a specified minimum temperature, or cooled to below a specified temperature, e.g., for storage or distribution. At present, the only useful technique is invasive probing by thermocouple or thermistor temperature sensors. The product so tested must be discarded after probing. Such a technique is therefore necessarily a sampling technique where the chosen samples are wasted. The temperature measured by probing therefore also only applies to a small volume of material around the probe, and the probing is subject to significant operator-dependent variability.
The present invention seeks to employ microwave radiometric temperature measurements which are inherently non-invasive and non-destructive. This allows potentially a whole production batch to be monitored, with no financial penalty, which can improve the efficiency of the process and the quality and safety control of the product.
It is desired that the microwave temperature measurement be made in a way that is independent of an operator, not be influenced by electromagnetic radiation external to the product being measured, and be dependent on the temperature within or throughout the bulk of the product material.
Electromagnetically enclosed cavities/cavity-antennas provide a way of coupling the microwave thermal radiation from a product to the radiation temperature measuring radiometer receiver that meets these requirements. The radiometric temperature of the signal from the cavity/cavity antenna must be measured by a microwave radiometer capable of measuring the temperature over the range required for the application, with an accuracy, resolution and response time appropriate to the application.
It is also desired to employ microwave radiometry requiring:
(a) Good microwave temperature measurement accuracy over an increasingly wide temperature range (comparable to good electrical thermometry), e.g., +/−0.5° C. from −20° C. to 100° C. for food product.
(b) High microwave temperature measurement accuracy over the bio-medical temperature range (e.g., +/−0.1° C. from 30° C. to 43° C.).
(c) An ability to determine the proper matched-impedance radiometric temperature of a source connected to the radiometer in the presence of impedance mis-match reflections between the source and the radiometer.
(d) An ability to make a determination of the proper matched-impedance radiometric temperature of a source connected to the radiometer with a uniform frequency response over the measurement bandwidth. This is highly desirable for measuring sources that may have a non-uniform radiation spectrum within the measurement bandwidth.
In one aspect the radiometer design requires to determine the source temperature independently of the gain of the microwave amplification, detection, and post-detection amplification. Strictly this may be the gain between a radiometer input (“Dicke”) switch and the post-detection synchronous demodulation. Implicitly the gain dependence contains frequency response independence (just gain at a given frequency). Gain stability and frequency response uniformity are, however, required between the input (“Dicke”) switch and the two reference sources. These are, however, passive microwave circuit paths and these requirements can be provided by practical microwave components.
It may also be desired to provide the following requirements:
(e) A need to measure radiometric temperatures at different frequencies to estimate temperature profiles within source materials.
(f) A need for radiometric temperatures measured at different frequencies to be accurately related to each other (application dependent but to less than 0.1° C. for medical applications and to less than 0.5° C. for industrial applications).
It is an object of at least one aspect of the present invention to obviate or at least mitigate one or more of the aforementioned problems.
It is a further object of at least one aspect of the present invention to seek to provide one or more of the aforementioned requirements.
The above-mentioned measurements need to be made with the best response-time temperature resolution that microwave radiometry is inherently capable of (i.e., The “Gabon limit”). For industrial applications the measuring radiometer must also operate in a wide range of ambient temperatures (e.g. from about −10° C. to about 40° C. without significant degradation of measurement accuracy). Further, the radiometer should not require frequent calibration.